The present invention relates to a differential device of the limited slip type for motor vehicles, by means of which drive to the two wheels of an axle of the motor vehicle can be transmitted whilst avoiding the possibility that when one of these wheels reaches the limit of adhesion it can accelerate sharply consequently unfavourably influencing the operating conditions of the motor vehicle or preventing this from moving with respect to the ground.
As is known, differential devices of this type normally comprise a pair of toothed planet wheels each of which is connected to an axle of the motor vehicle and which are connected together by means of a kinematic chain in which there is at least one pair of toothed wheels of low inherent efficiency or a pair of toothed wheels which do not allow reverse drive; normally this pair is constituted by a worm screw and a helically toothed gear wheel.
A first differential device of this type, known as a Torsen differential, comprises a pair of worm screws each of which constitutes a toothed planet wheel of the differential, and three pairs of helically toothed satellite wheels each of which is carried by the differential housing and is rotatable with respect thereto on an axis which lies in a plane orthogonal to the axis of the toothed planet wheels. Each of the said helical gears of the pair mesh with a corresponding worm gear and is fixed to at least one cylindrically toothed wheel which in turn meshes with the cylindrically toothed wheel to which the other helically toothed gear wheel of the same pair is fixed.
With this constructional arrangement, when the vehicle runs in a curve, the wheels can assume a different angular velocity since each worm gear (to which one of the wheels is fixed) is able to transmit drive to the corresponding helical gear, whilst sharp acceleration of a wheel when this reaches the limit of adhesion is prevented since, if this anomalous condition of operation should occur, one of the two worm gear-helical gear pairs must rotate with reversed drive, which is prevented by the geometry of the teeth of the pair itself.
Differential devices of this type have several disadvantages.
First of all, very high torques cannot be transmitted with these because of the known stresses which are produced in some regions of the teeth of the gear pairs and the differential housing. In fact, torque is transmitted from the differential housing to each of the axles through the forces which are exchanged between each worm gear fixed to the associated axle and the corresponding helical gears: this torque cannot be very high because of the limited number of meshing zones between each worm gear and the corresponding helical gear (three meshing zones) and the small contact areas in each of the said zones. Moreover, high stresses are also manifest at the seats of the differential housing in which the toothed satellite gears are rotatable because of the limited dimensions of the seats themselves and the form of the casing imposed by the structure of the mechanism. The radial dimensions of such a differential device are very high because of the arrangement of the axes of the toothed satelite wheels which, as has been stated, lie in planes orthogonal to the axis of the differential.
Finally, the operations necessary for assembly of such a device are complex and require particulare care.